The parallels here are set up by the repetition of the word day. The idea that Jehovah has heard Nephi’s cry by day is parallel to his waxing bold in prayer during the day. Both of these phrases have the same meaning and the same intent, but the repetition with different words intensifies the meaning.
The second part of the parallel is what happens because of the cry by day, or the mighty prayer by day. The first is that Nephi received revelation. The idea that visions in the night are equivalent to revelation is also shown in Genesis 46:2, which says that “God spake unto Israel in the visions of the night.”
The result of the mighty prayers sets up an antithetical parallel. The prayers are sent up on high, and angels come down to minister to Nephi.